Solid Oxide Fuel Cell (SOFC) power generation systems capable of operating on coal derived and hydrocarbon fuels (e.g., natural gas, diesel, etc.) are being developed for stationary and mobile land based applications. Several SOFC prototype demonstration units, ranging in sizes from 3 kWe to 25 kWe, have been fabricated and field tested using hydrogen and water gas mixtures and natural gas as fuels.
SOFC power generation systems offer lower stack pollution levels in the exhaust gas stream due to the electrochemical oxidation of fuels at relatively lower cell operating temperatures, which reduces NO.sub.x emissions, and due to the use of clean sulfur free fuels, which reduces SO.sub.x emissions. Such systems also provide higher power conversion efficiency (kWeH/MBTU of fuel) in comparison with other types of power generation systems. SOFC systems may also be of modular construction, making the systems ideal for various power generation applications.
Examples of SOFC systems are disclosed in U.S. Pat. No. 4,395,468 to Isenberg, U.S. Pat. No. 4,702,971 to Isenberg, U.S. Pat. No. 5,143,800 to George et al., U.S. Pat. No. 5,306,574 to Singh et al. and U.S. Pat. No. 5,413,879 to Domeracki et al., each of which is incorporated herein by reference.
The long term successful operation of SOFC generators depends primarily on maintaining structural and chemical stability of fuel cell components during steady state conditions, as well as transient operating conditions such as cold startups and emergency shut downs. During steady state operation, nickel-containing cell fuel components such as electrodes and contact members (e.g., nickel felt contacts for cell to cell and cell to bus bar connections) are exposed to a fuel gas atmosphere in which nickel remains thermodynamically stable as nickel metal. During such steady state operation, the oxygen pressure of the fuel gas is lower than the Ni/NiO equilibrium oxygen pressure. The SOFC air electrode, typically made of doped lanthanum manganite, similarly remains chemically and structurally stable in the surrounding air atmosphere during steady state conditions.
Under transient operating conditions such as cold startups and emergency shut downs, non explosive N.sub.2 --H.sub.2 gas mixtures (typically a N.sub.2 -3% H.sub.2 gas mixture) known as "NH mix" cover gases are conventionally used in SOFCs to preserve and maintain the chemical stability of the nickel fuel electrode and nickel felt connections. In addition, hydrogen-rich gas streams have been used in SOFC's during startup current loading prior to switching to the primary fuel, such as natural gas. On-site gas storage of both NH mix and H.sub.2 gases has been required for conventional systems. However, storage of gas cylinders in the proximity of SOFC generators requires a large amount of space and, in the case of H.sub.2, elaborate safety measures for the prevention of explosion. For larger SOFC systems where NH mix and hydrogen-rich gas requirements are expected to be very large, storage of gases will put even a higher demand on safety and space requirements. This scenario does not appear attractive for situations where gas storage space and accessibility is limited. The above arrangements may also prove very expensive (higher gas costs, cylinder leasing expenses, transportation expenses, etc.) for the operation of the generators.
A system for on site generation of non explosive N.sub.2 --H.sub.2 cover gas and H.sub.2 -rich startup gas would be highly advantageous during the startup and cool down of SOFC power generation systems. Such a system would represent a major improvement over on site storage of N.sub.2, H.sub.2 or N.sub.2 --H.sub.2 blend gas cylinders. The present invention has been developed in view of the foregoing.